The invention relates to an electric motor controller and an electric motor controller for a vehicle steering apparatus.
A brushless motor is used as a drive source that supplies a driving force to, for example, a vehicle steering apparatus. An example of the vehicle steering apparatus is an electric power steering apparatus. An electric motor controller that controls driving of the brushless motor is usually configured to control the electric current that is supplied to the motor based on the output from a rotational angle sensor that detects the rotational angle of a rotor. As the rotational angle sensor, a resolver that outputs a sine-wave signal and a cosine-wave signal that correspond to the rotational angle (electrical angle) of a rotor is usually used. However, the resolver is expensive, and needs a large number of wires and a large installation space. Therefore, using the resolver as the rotational angle sensor hinders reduction in cost and size of a unit that includes the brushless motor.
U.S. Patent Application Publication No. 2007/0229021 A1 describes a sensorless drive method for driving a brushless motor without using a rotational angle sensor. According to the sensorless drive method, the induced voltage that varies depending on the rotational speed of a rotor is estimated in order to estimate the phase of a magnetic pole (electrical angle of the rotor). When the rotor is at a standstill or rotating at a considerably low speed, it is not possible to estimate the induced voltage. Therefore, the phase of the magnetic pole is estimated by another method. More specifically, a sensing signal is input in a stator, and a response of the motor to the sensing signal is detected. Then, the rotational position of the rotor is estimated based on the response of the motor.